Epoxy novolac resins contain a phenol-formaldehyde backbone and multiple epoxy groups attached to the backbone. The chemical structure of epoxy novolac resins offers several advantages in comparison with conventional bisphenol A (BPA) or bisphenol F (BPF) type epoxy resins, particularly with regard to solvent and chemical resistance in cured films. Thus, epoxy novolac resins have been widely used in tank linings and chemical storage tanks; such tanks require a higher demand on coating performance (related to solvent and chemical resistance) that cannot be provided by conventional BPA or BPF type epoxy resins. For example, an epoxy novolac resin commonly used for tank linings is DEN 438, an epoxy novolac resin commercially available from The Dow Chemical Company. However, typical epoxy novolac resins used for tank linings and chemical storage tanks usually have a high viscosity which results in a low solids content (for example, a volume solids of 50 percent (%) or less) and a high VOC (volatile organic compounds) (for example, a volume solids of greater than 50%) in coating formulations produced from such epoxy novolac resins such as when used in coatings and paints. Some of the epoxy novolac resins commercially available from The Dow Chemical Company and the resins' melt viscosities include for example:
Epoxy Novolak ResinRange of Melt ViscosityD.E.N. 431 1,100-1,700 mPa · s @ 52° C.D.E.N. 43831,000-40,000 mPa · s @ 52° C.D.E.N. 43915,000-35,000 mPa · s @ 71° C.
Driven by increasing awareness of environment protection from governments, paint formulators, and end users, currently there is a strong demand in the industry for high solid coating formulations (for example, a volume solids of greater than 50%) with low VOC (for example, a VOC of about 420 g/L or less). Moreover, high solid paint products could also give customers economic benefit by decreasing the amount of solvent usage in formulations and by simplifying the process of applying the formulations. A typical tank coating usually is made up of three epoxy novolac layers with a DFT (dry film thickness) of approximately (˜) 100 μm for each layer. High solid paint can increase the DFT of each layer and may provide the capability of using a two-layer or even a one-layer coating system instead of the current three-layer coating system without sacrificing coating performance. Reducing the number of layers required for tank coatings from three coating layers may also greatly decrease coating application costs and increase coating application productivity.
Heretofore, many attempts have been made by those in the industry to increase the solid content of coating formulations and to achieve a high DFT for a single layer. For example, one approach is to use standard BPA or BPF type epoxy resins such as DER 331 and DER 354—bisphenol A/F type epoxy resins commercially available from The Dow Chemical Company. However, when using a bisphenol A/F type epoxy resin a thick film (for example, about 300 microns) is required to obtain adequate performance for use in coatings for storage tanks. In addition, the thick film based on bisphenol A/F epoxy resins usually has strong internal stresses, which easily result in formation of cracks in the film during curing and during the service period of the film. It is known to those skilled in the art that the higher the DFT of a layer, the higher the internal stresses in the layer; and it is difficult for the end user, such as paint applicators, to balance film thickness and crack resistance.
Another approach in an attempt to achieve high solids content in a coating formulation is to use, in the formulation, conventional cycloaliphatic amine or aromatic amine hardeners that have a low molecular weight and a low viscosity such as bis-(p-aminocyclohexyl) methane (PACM), 1,2-cyclohexane diamine (DACH), diethylenetriamine (DETA), meta-xylene diamine (MXDA) and the like. However, the low molecular weight amine hardeners have poor compatibility with epoxy novolac resins and need a long induction time. The poor compatibility problem is indicated when blushing occurs in the final film product. In addition, the poor compatibility property eventually leads to poor coating properties. Therefore, to avoid the above problems, a long induction time (for example, greater than 30 minutes) is necessary when low molecular weight amine hardeners are used. It is also difficult for the end user, such as paint applicators, to balance an acceptable induction time (for example, 15-30 minutes) and pot-life (for example, 1-2 hours) of coating formulations during application of the coating formulation.
WO 2009/126393 A2 discloses low temperature cure epoxy resin compositions and a process for preparing such epoxy resin compositions. The examples of WO 2009/126393 A2 describe a novolac epoxy resin, DEN 438, modified with 10%, 15% and 20% dodecylphenol; and with 9.5% 3-pentadecenylphenol (mole % phenol group per epoxy group), respectively. In coating formulations, the modified novolacs are cured with a phenalkamine type hardener; and the resultant coatings show a faster curing property, a non-sticky film surface property and a good appearance property, at both room temperature (e.g., 23° C.) and low temperature (e.g., 0° C.) curing conditions, than an unmodified DEN 438. Although WO 2009/126393 A2 does not provide any epoxy resin viscosity measurements, it has been found that a dodecylphenol modified novolac resin has a higher viscosity (for example, greater than 33,000 mPa·s at 70° C.) than DEN 438.
KR515624B1 discloses a weak solvent-soluble epoxy resin, a preparation method for such epoxy resin, and a paint composition with better corrosion resistance and chemical resistance than conventional BPA or BPF type epoxy resins. In general, the weak solvent-soluble epoxy resin of KR515624B1 is prepared via the following steps: (1) reacting an alkyl phenol having an alkyl group of C8 or more and an aldehyde to prepare a novolac resin; (2) reacting the novolac resin with an epoxy resin, wherein the ratio of the OH groups of the novolac resin to the epoxy group is 0.4-0.6 to 1 by equivalence; and (3) reacting the obtained reaction product from step (2) with ⅙ to 1/12 equivalence of an aliphatic acid per the epoxy group by addition reaction.
KR515624B1 further discloses that the alkyl phenol used is at least one selected from p-octylphenol, nonylphenol, p-dodecylphenol and cardanol; the aldehyde is formaldehyde, p-formaldehyde or acetaldehyde; and the epoxy resin is bisphenol A epoxy, bisphenol F epoxy, or a bifunctional polyglycidyl ether.
KR515624B1 describes a synthesis process for preparing an epoxy resin product reacting an alkyl phenol and an aldehyde, and then subsequently reacting the above resultant reaction product with an epoxy resin. The obtained product is then further reacted with an aliphatic acid to generate the final epoxy resin product. The alkyl phenols of KR515624B1 have an alkyl group of C8 or more and the epoxy resin of KR515624B1 is bisphenol A, bisphenol F, water-addition bisphenol A or a bifunctional polyglycidyl ether. KR515624B1 provides a complex process for producing a coating product with a structure different from coatings produced from typical epoxy novolac resin formulations.
“Preparation and Anticorrosive Performances of Polysiloxane-modified Epoxy Coatings Based on Polyaminopropylmethylsiloxane-containing Amine Curing Agent”, J. Coat. Technol. Res., 8 (4), pp. 481-487, 2011 is an article that discloses preparing polysiloxane-modified epoxy coating formulations and using cardanol in the coating formulations as a compatibilizer. Cardanol is found to increase the miscibility of a hardener with epoxy resins and to improve the coating performance of formulations using the cardanol. In the above article, the cardanol is cold blended into the coating composition to act as compatibilizer. However, the final composition prepared by the process disclosed in the above article cannot be used to prepare a coating with adequate chemical resistance for use in a tank coating application.